1. Field of the Invention
This invention relates to an image display device such as a television image receiver including a wide aspect ratio screen, and more particularly to an image display device adapted to display, on the wide aspect ratio screen, an image of a video signal to which picture quality adjustment processing is implemented by picture quality adjustment means to which either an input video signal of a standard aspect ratio or an input video signal of a wide aspect ratio is input.
2. Description of the Related Art
Hitherto, image display devices, e.g., television image receivers for displaying an image by a video signal with display means such as a cathode ray tube (CRT) or a liquid crystal display have been widely known. In such image display devices, a technique is employed to implement picture quality adjustment processing such as a sharpness adjustment or a contrast adjustment to a video signal, thereby making it possible to provide a display image having excellent picture quality by using display means such as the above-mentioned CRT or the like.
Meanwhile, in the existing television broadcasting, the lateral-to-longitudinal ratio (aspect ratio) of the display screen is set to 4:3. However, in an EDTV (extended definition TV) broadcasting of the second generation which will be put into practice in the future, a high definition television broadcasting such as a high vision or the like, planning is made to widen the aspect ratio of the screen so that it is equal to 16:9.
Accordingly, it is foreseen in the future that the television broadcasting of the wide aspect ratio of 16:9 will be conducted in addition to the existing television broadcasting of the standard aspect ratio of 4:3. For this reason, image display devices capable of displaying images of both video signals are being developed at present.
In such image display devices, as the system in which e.g., display means of the wide aspect ratio of 16:9 is used to display an image of a video signal of the standard aspect ratio of 4:3, the following systems have been proposed.
Namely, the first system is, as shown in FIG. 1A, the system called a standard mode to implement a processing such that a picture P.sub.1 of a video signal of the standard aspect ratio falls within the wide aspect ratio screen to mask both left and right side regions P.sub.2 and P.sub.3. In this standard mode, a video signal of the standard aspect ratio is subjected to time base compression so that it is reduced to 3/4 in a horizontal direction in correspondence with a difference of the aspect ratio with respect to a display picture of the wide aspect ratio to add, in the both the left and right side regions of that video signal, mask signals displayed, e.g., by black, thus to effect display in accordance with deflection of the same raster scanner as in the ordinary case.
Further, the second system, as shown in FIG. 1B, called a top and bottom cut mode in which respective top and bottom regions P.sub.4 and P.sub.5 of an image of a video signal of the standard aspect ratio are cut, but the image P.sub.6 is displayed on the entire with of the display screen. In the case of the top and bottom cut mode, over scan is applied to deflection in the upper and lower directions without processing the video signal of the standard aspect ratio to effect display.
Meanwhile, it is now assumed that, in an image display device adapted to display an image of the wide aspect ratio and an image of the standard aspect ratio by using a wide aspect ratio screen, an image of the standard aspect ratio is displayed on the wide aspect ratio screen in the above-described standard mode or top and bottom cut mode. Under this circumstance, even in the case where an image of an input video signal of the wide aspect ratio is satisfactorily displayed, the picture quality of other images may be disadvantageously degraded for the reason described below.
Namely, in order to display an image by a video signal of the standard aspect ratio on the wide aspect ratio screen in the standard mode as shown in FIG. 1A, it is necessary to implement time base processing to compress the time base of a video signal so that it is reduced to 3/4 in a horizontal direction. A display image in the standard mode is such that the frequency component of a video signal is 4/3 times larger than the original one by the time base processing, resulting in insufficient sharpness of picture.
When it is assumed that an image from a video signal of the standard aspect ratio is displayed on the wide aspect ratio screen in the top and bottom cut mode as shown in FIG. 1B, the number of effective scanning lines serving as a display picture changes from 480 to 360. For this reason, the luminance of the picture is reduced to 3/4, resulting in a darker picture.